In most microwave tubes the interaction between the wave and the beam is divided into two steps:                a first step in which electrons are grouped into bunches, i.e. in which the density of the current of the beam is modulated depending on the frequency of the microwave signal; and        a second step in which the bunches of electrons thus obtained are placed in a phase in which they are slowed by the field in order to transfer their energy to the wave.        
In the case of TWTs, the electrons are grouped into bunches by placing the beam in the field of a travelling wave the phase velocity of which is equal to the velocity of the electrons. In a moving coordinate system, the electrons see the field of a stationary wave. The electrons are slowed down in one half-wave and accelerated in the following one. A bunch of electrons forms around the phase for which an accelerating field changes to a decelerating field.
A conventional waveguide, of rectangular or cylindrical cross section, is not suitable for this type of interaction because the phase velocity of the wave that propagates through this waveguide is higher than the velocity of light whereas the velocity of the electrons is lower than the velocity of light. In addition, an electric field parallel to the movement of the electrons is required whereas the fundamental mode of rectilinear waveguides of rectangular or cylindrical cross section is perpendicular to the axis of the waveguide. To obtain a phase velocity lower than that of light a special waveguide called a slow-wave structure or delay line is required. Most often, the delay line is a periodic line obtained by repetitively translating a basic cell, in order to obtain a succession of identical cells. This is the case for helix TWTs, coupled-cavity TWTs, interdigital-line TWTs, etc.
In the field of TWTs operating at millimeter wavelengths a folded-waveguide delay line is often used. This type of line is obtained by periodically positioning sections of rectangular waveguide perpendicular to the axis of the beam, and by alternatively connecting the sections of straight waveguide with bends generating 180° E-plane rotations. Seen from the side, the folded waveguide is serpentine-shaped. The beam tunnel is located in the middle of the straight sections of rectangular waveguide. The electric field in the waveguides is perpendicular to the broadside of the waveguide, and therefore parallel to the movement of the electrons, thereby allowing the beam to be modulated. The electrons therefore move through the beam tunnel, enter into a straight waveguide section, where they experience the action of the electric field (interaction space), pass back through the beam tunnel and enter into the following interaction space. The electrons therefore see the successive interaction spaces with a period equal to the pitch of the line, whereas the geometric period of the line is equal to two times the pitch. The pitch corresponds to the distance between two straight waveguides separated by a bend.
The length of the folded waveguide (straight portion and bends) is determined so that the phase shift of the wave in the waveguide corresponds to the phase variation related to the movement of electrons from one interaction space to the next.
Travelling-wave tubes use a delay line including a number of sections higher than or equal to 2. The input section is terminated by a load and the output section starts with a load. Intermediate sections start and end with a load. The term “load” is understood to mean a volume containing a material that absorbs RF waves, connected to the delay line such that, in the connection plane, the impedance presented by the volume is as close as possible to the characteristic impedance of the delay line so as to ensure a good match (i.e. to minimize the wave reflected by the load).
FIG. 1 schematically shows a slow-wave structure or delay line for a travelling wave tube comprising three sections 1, 2 and 3. The delay line shown comprises an input 4 and an output 5.
The loads 6 at the output of the first section 1, at the input of the second section 2, at the output of the second section 2, and at the input of the third section 3 are called sever loads. Between the end of one line and the start of the following one the electron beam passes through a beam tunnel in which the RF wave does not propagate, and as a consequence has no bunching action, this contributing to debunching of the beam (this is therefore a loss of modulation).
If the reflection coefficients at the two ends of a section and the gain of the section are too high, an oscillation may be observed in this delay-line section. For this reason, the length of the various sections is determined so as to limit gain, on account of the reflection coefficient of the sever loads.
The most commonplace TWTs, an example of which is illustrated in FIG. 2, use a delay line comprising a helix 7 that is held in an envelope 8 by three dielectric rods 9.
In a delay line of the type in FIG. 2, loads are generally produced by depositing, on the rods 9 supporting the helix 7, a layer of lossy material, such as graphite. A lossy material is characterized by a finite electrical conductivity G (in contrast to a perfect conductor the electrical conductivity of which is infinite), resulting in a conduction current σE (E being the electric field) and resistive losses σE2. In a lossy medium the wave undergoes an exponential attenuation as a function of distance. By varying the thickness of the deposit, a load is produced the attenuation (microwave loss) and reflection coefficient of which increase gradually, thus allowing a good match to be obtained over a wide frequency band.
In such a helix delay line, the length of the load leads to a substantial loss of modulation, and therefore to a decrease in the gain of the TWT, which it is necessary to compensate for by increasing the gain of the other sections and therefore the total length of the TWT.
FIG. 3 schematically shows the attenuation on a rod 9 as a function of the thickness z of the deposit of the lossy material, such as graphite. The higher the attenuation, the darker the grey colour that represents it.
In the case of a TWT using a folded-waveguide delay line, it is known to interrupt the modulations in order to pass from a folded waveguide 10 to a straight waveguide 11 in which a load that absorbs electromagnetic energy is placed. Such a straight waveguide may either be parallel, as shown in FIGS. 4 and 5, or perpendicular, as shown in FIG. 6, to the beam tunnel 12.
In such embodiments, although the same waveguide cross section is used, the periodic folded-waveguide line and the straight waveguide containing the load do not have the same impedance and it is necessary to insert a matching circuit at the transition from one line to the other, which is not wide band, and limits the bandwidth of the TWT.
As a variant, as shown in FIG. 7, it is known to interrupt the folded-waveguide delay line in order to allow insertion of a lossy dielectric block 13 of a geometry determined to minimize the reflections of the load.
This variant comprises an abrupt transition between the periodic folded waveguide 10 and the lossy dielectric block 13, which is equivalent to loading the periodic folded waveguide 10 with a lossy resonator possessing many resonances, this limiting the frequency band in which the load is well matched.